GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Audio Play Icon Headphones Icon Plus Icon Minus Icon Check Icon Print Icon Picture icon Single Arrow Icon Double Arrow Icon Hamburger Icon TV Icon Close Icon Sorted Hamburger/Search Icon
Building Science

Is Compressed Fiberglass Insulation Really a Problem?

Or is this just another myth in the world of building science?

Good installation of fiberglass batt insulation is critical. But how important is compression?
Image Credit: Energy Vanguard

I’ve been guilty of perpetuating a myth. Not long ago I wrote an article in which I said installing insulation, “cavities [should be] filled completely with as little compression as possible.” But is compression really such a bad thing? Here on GBA, commenter Dana Dorsett wrote, “Compression of batts is fine (resulting in a higher R/inch due to the higher density) as long as the cavity is completely filled.”

He’s right. Compression isn’t the problem. Incompletely filled cavities are a problem. Gaps are a problem. But you can compress fiberglass insulation quite a bit and it still works just fine. The North American Insulation Manufacturers Association (NAIMA) has a little two-page document about compressing fiberglass insulation. Here’s what NAIMA says: “When you compress fiber glass batt insulation, the R-value per inch goes up, but the overall R-value goes down because you have less inches or thickness of insulation.”

The document includes a general chart for how to tell what your R-value is with different levels of compression. Owens Corning also has a compression chart for R-value (see Image #2, below).

So, you don’t get the full R-value on the label, but the insulation still works perfectly well if all you’ve done is compress it. Of course there are limits. If you use a hydraulic press to compress it so much that it approaches the density of solid glass, things change. We’re talking about reasonable amounts of compression.

Here’s something you may not know. The standard R-19 fiberglass batt is 6.25 inches thick. If you put that batt in a closed 2×6 wall, it will be compressed 0.75 inch because a 2×6 is 5.5 inches deep. That means the batt labeled R-19 really gives you R-18 in a closed cavity.

One place where you’re pretty…

GBA Prime

This article is only available to GBA Prime Members

Sign up for a free trial and get instant access to this article as well as GBA’s complete library of premium articles and construction details.

Start Free Trial

0 Comments

Log in or become a member to post a comment.

Related

Community

Recent Questions and Replies

  • |
  • |
  • |
  • |